It is known that the electric furnaces for smelting of iron based scraps being directly discharged from the top have a front opening, also called "slag hole" whose threshold or lower step forms the so-called "dam" suitable to keep the molten bath within the furnace and to be overflown by the slag only, which then is drained at an underlying level through a trap provided on the floor immediately at the outside of the slag hole. The latter also provides the passage for the oxygen lance through which oxygen gas is blown at very high speed directly into the melting zone for refining the melt.
Also some important inconveniences are known, in connection said slag hole and with the presence of mainly deriving from the top charge of the scrap, which causes the obstruction, at least partial, of the so-called "slag runner" which connects the outer mouth of the hole with the molten bath and must remain unobstructed, in particular to allow the oxygen lance without obstacles.
On the other hand it is also known that usually it is necessary to await that a cast is over to carry out, before the subsequent one begins, the so-called operation of "slag hole cleaning", in other words removing the slag, now cooled, that has deposited on the threshold of said hole, as well as remaking the "dam". Of course this involves longer standstill times between one cast and another, with consequent reduction of the production capacity, since it is impossible to carry out these operations by hand with the electrodes being fed, near high temperatures and with the operators under the danger of being hit by sparks or jets of molten metal. Not only, but the removal of eventually cooled, solid slag, forming an integral block with the underlying step of refractory material (usually dolomite), resulted in a substantial demolition of the step itself or "dam" and its remaking, with an additional extension of the plant standstill time.
It has been thought of over coming these inconveniences by using a power shovel that can hold the batch of charged scrap at the inside of the furnace while keeping the slag runner free, but the required presence of an operator on board renders extremely dangerous and nearly impossible this solution unless the operator of the mechanical shovel undergoes unlikely performances such as the temporary leaving of the vehicle as the material drops. Similarly unfeasible results to be the cleaning of the slag hole with active electrodes, when considering the high electric power involved (up to 65 MW).
Therefore the object of the present invention is that of providing an apparatus capable of carrying out the above-mentioned operations, under a remote control and without the presence of an operator on place.
With the pushing machine according to the present invention, during the scrap charge into the furnace a first series of advantages is obtained, among which the most apparent one is that of preventing the scrap from escaping the slag hole, thus increasing the yield, i.e. the ratio between produced steel and charged scrap. Another important advantage in this stage is that the slag runner, as above defined, is kept free and the scrap drops entirely within the furnace, not on the threshold thereof, where it would be lost, thus reducing even more the yield, and wherefrom it should have to be removed with difficulty once solidifyed, with a partial demolition of the threshold itself which then should have to be restored. Furthermore the completely free slag runner means immediate exploitation of the oxygen in the zone where melting occurs, without danger of damaging the lance and anyhow without possible presence of scrap between lance and melt bath which would interfere with the oxygen blow, thus reducing its speed and shielding the temperature gradient.
Another series of advantages afforded by the pushing machine according to the present invention is due to the fact that it can be used for cleaning the slag hole and remaking of the dam at about half casting, when for example the electrodes have delivered a prefixed quantity of specific power (such as for example 200 KWh/ton) and are still active. Thus the power delivery is not interrupted and the standstill time between two subsequent casts is strongly reduced, while the slag overflow level is ensured, as the height of the head shield, being controlled to have the fixed values, keeps automatically constant such a level, thus slag is removed when it is hot, thereby extremely crumbly, without problems of re-building the dam with additional dolomite, all this occurring without any personal intervention, what would clearly involve some risks.